firmware: arm_scmi: Remove zero-length array in SCMI notifications

Substitute zero-length array defined in scmi_base_error_report with
a flexible length array definition.

Link: https://lore.kernel.org/r/20200710133919.39792-1-cristian.marussi@arm.com


Signed-off-by: Cristian Marussi <cristian.marussi@arm.com>
Signed-off-by: Sudeep Holla <sudeep.holla@arm.com>